1. Field of the Invention
The extraction of heavy metals from the aqueous solutions of the salts which contain them, by electrolysis (electrowinning), requires the use of insoluble anodes which are good electrical conductors, and, simultaneously, are endowed with a high enough resistance to the electrolyte used and to the products from the relevant anodic reactions, and, finally, favor the development of oxygen.
2. Discussion of the Related Art
For those metals which are more commonly produced by this route: copper, nickel, manganese, zinc, cadmium, and so forth, the present art preferably uses anodes of bonded lead (with antimony, silver, calcium, and so forth).
In the usual sulfuric baths used to electrowin the above metals, the lead anodes get coated with a thin layer of lead sulfate which, by oxidation, is transformed into a layer consisting of Pb dioxide, which protects them from further corrosion and which, by being conductive, uses the development of O.sub.2 with a suitably low oxygen overvoltage.
In order to electrowin copper and nickel from the solutions of their sulfates, anodes have been used for many years, which consist of lead containing 6-8% of Sb, and which are consumed very slowly, unless chloride ions are present in the electrolyte. Unfortunately, the anode of Pb/Sb does not prevent Pb from polluting the cathode.
On the contrary, for zinc electrowinning, anodes of Pb with 0.5-1% of Ag are used, which are obtained by casting or lamination, and sometimes are provided with grooves to favor oxygen development and other times are provided with circular holes to favor the circulation of the electrolyte. The conduction of electrical current throughout the anode is secured by inserting the copper bar inside the body of the same anode, by melting. The resistance of these anodes to chemical attack by the electrolyte is undoubtedly good, and the useful operating life of such electrodes is often longer than 2-3 years.
A negative characteristic consists in that, owing to the presence in the zinc-containing solution, of a certain level of manganous ions, adhering scales of MnO.sub.2 are formed on the anode, which become thicker and thicker with time.
When these scales get detached, owing to natural processes, they release particles of PbO.sub.2 and/or of PbSO.sub.4, which increase the Pb level in the cathodic zinc.
Another common problem displayed by the lead anodes used in the above cited electrolyses, is the large amount of immobilized metal (the weight of an anode of the cells known from the prior art is always higher than 100 kg), and the costs deriving from the periodic restoration of the anodes. Furthermore, in many facilities, there is burdensome maintenance, due to the periodic removal of the scales (every 2-4 weeks), which is carried out in order to improve the quality of produced zinc, and reduce the cell voltage.
The production of lead by the electrolytic route is presently the focus of interest of the big metallurgical industry: the fluoroboric and fluorosilicic electrolytes, preferred owing to the higher quality of deposits which can be obtained, causes serious problems of resistance of the anodic material to arise.
E. R. Cole et al., U.S. Pat. No. 4,212,340, uses an anode constituted by a titanium sheet electrolytically coated with a thin-texture layer of PbO.sub.2, with a particularly compact structure.
M. Ginatta, U.S. Pat. No. 4,098,658, uses anodes made from graphite bars, which get naturally coated with PbO.sub.2, and are sheltered by it.
R. D. Prengaman et al., U.S. Pat. No. 4,236,978 uses anodes made from a graphite plate wrapped in a net made from a plastics material, which serves as a reinforcement for the deposit of PbO.sub.2, and counteracts the brittleness thereof.
All these types of anodes display a poor electrical conductivity, are rather brittle, and their useful operating life is rather short.
Also, the problems caused by the anodic materials used to produce oxidizer halogenated salts (at present, activated Ti or Pt are used), are not completely solved.